This invention relates to a method for producing a black colored steel strip.
Black colored steel strips are finding wide application in household appliance, copying machine, communication equipments, automobile components, and interior building materials. Conventional processes for producing such black colored steel strips include (1) black film coating, (2) chemical treatment, (3) black chromate treatment, and (4) anodic treatment. Among these, anodic treatment is known to results in a steel strip having improved degree of blackness.
Japanese Patent Publication Nos. 61-60915, 63-46158, and 63-46159 relate to a production of black colored steel strips by subjecting a zinc alloy-plated steel strip to anodic electrolysis in an electrolyte containing sulfate, nitrate, or the like.
The black colored steel strips prepared by such an anodic electrolysis have high degree of blackness as well as excellent appearance when they are prepared in a freshly prepared electrolyte.
During the anodic electrolysis, metals including zinc dissolve out of the zinc alloy-plated layer. The dissolved metals, when they reach certain concentration, form white corrosion products which deposit on the steel substrate, leading to a deteriorated degree of blackness as well as nonuniform appearance. Therefore, in a continuous line employing the conventional electrolyte solution, color tone of the resulting black colored steel strip varied with an increase in the amount of the steel strip being treated.
In order to produce uniformly black colored steel strips in the above-described bath, various supplemental processes including constant addition of the freshly prepared electrolyte solution and continuous zinc removal were necessary. Such supplemental processes suffered from various critical defects for an industrial-scale production including difficult process control as well as increased production cost.
There are strong demands for uniformly colored black steel strips having a high degree of blackness expressed in L value of less than 15 in the field of household appliance, copying machine, and the like. The aforementioned prior art anodic electrolysis, however, was insufficient for preparing such black colored steel strips required in these applications.
The inventors of the present invention have made an extensive investigation and found that an incorporation of an inhibitor primarily comprising a nitrogen compound, a sulfur compound, an amine, or a halogen compound into the electrolyte solution prevents the deposition of zinc-containing corrosion products onto the surface of the steel strip, thereby improving the degree of blackness of the resulting products. It has also been found that an incorporation of a complexing agent such as citric acid, EDTA, oxalic acid, and tartaric acid prevents the formation of the corrosion products, again improving the degree of blackness.